A. Field of the Invention
The present invention relates generally to pneumatic conveying systems and more particularly to an elbow for use in such systems that minimizes or reduces the formation of fines and streamers during the conveyance of pelletized plastic products.
B. Description of the Prior Art
Pelletized plastic products are typically conveyed pneumatically through piping systems. The plastic pellets travel through the systems at relatively high velocities and the pellets are subject to degradation due to heat of friction generated as the pellets come into tangential contact with the pipe wall.
One form of material degredation is the fomation of "fines", which involves the chipping away of small pieces of the plastic product. A second form of degredation is the formation of "streamers", which occurs when the plastic particles slide along in contact with the pipe wall. Since the velocities are high and the melting point of the plastic particles is relatively low, the sliding of the partical along the wall actually causes the plastic at the point of contact to melt and leave a smear of plastic on the pipe wall. The smears build up over time into a long sheet of plastic. Occasionally, the sheets come loose from the walls of the pipe and form streamers.
There have been developed certain methods for decreasing the formation of fines and streamers. Most commonly, such methods involve treating the inside of the pipe to cause particles striking the pipe wall to be deflected back toward the centerline of the pipe. One method of treatment is disclosed in Avery, Jr. et al, U.S. Pat. No. 4,230,426, which cites additional patents and methods in the field.
The above described methods of pipe treatment have been at least somewhat effective in reducing the formation of fines and streamers in straight pipes. However, pneumatic conveying systems typically include bends in the pipes. These bends are normally formed by long radius elbows. Pipe interior surface treatment methods have been applied to such elbows but have not been as effective as in the case of straight pipes.
The primary problem with long radius elbows, as regards product degredation, is in the fact that the particles tend to travel along the wall of the pipe on the outside of the bend. The particles either slide along the wall, thereby leading to the formation of streamers, or bounce repeatedly along the pipe surface, thereby leading to the formation of fines. An additional shortcoming in the use of long radius elbows is in the energy loss due to frictional contact of the product with the pipewall in the bend. Additionally, long radius elbows are difficult to build and install in piping systems. Also, the elbows tend to wear out faster than do the straight pipes in the systems, and therefore, have to be replaced relatively frequently, which leads to increased system down time.
It is therefore an object of the present invention to provide an elbow for use in pelletized plastic pneumatic conveying systems that overcomes the shortcomings of the prior art. More specifically, it is an object of the present invention to provide an elbow for use in pneumatic conveying systems that reduces product degredation. It is a further object of the present invention to provide an elbow for use in pneumatic conveying systems that decreases energy loss. It is a further object of the present invention to provide an elbow for use in pneumatic conveying systems that has decreased life expectancy over those of the prior art. It is yet a further object of the present invention to provide an elbow for use in pneumatic conveying systems that is easier to build and install.